Experiments upon magnesia alba, quick-lime, and other alcaline substances / by Joseph Black.

  • Black Joseph, 1728-1799.
Date:
1893
Catalogue details

Licence: Public Domain Mark

Credit: Experiments upon magnesia alba, quick-lime, and other alcaline substances / by Joseph Black. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    When we consider that the attraction of alkalis for fixed air, is weaker than that of the calcareous earths, and reflect upon the efTects of heat in chemistry, we are led to imagine, that alkalis might be entirely deprived of their air, or rendered perfectly caustic, by a fire some- what weaker than that which is sufficient to produce the same change upon hme; but this opinion does not seem agreeable to experience. The alkalis do, however, acquire some degree of causticity in a strong fire, as appears from their being more easily united with spirit of wine, after having been kept in fusion for some time. For that fluid, which cannot be tinctured by a mild salt of tartar, will soon take a very deep colour from a few drops of a strong caustic-ley. The circumstances which hinder us from rendering these salts perfectly caustic by heat, are, their propensity to dissipation in the utmost violence of the fire, their extreme acrimony, and the imperfection of our common vessels. For, before the heat becomes very intense, the alkalis either evaporate, or dissolve a part of the crucibles in which they are contained, and often escape through their pores; which happens, especially, as soon as they have already acquired some degree of additional acrimony, by the loss of part of their air. The fusion, also, which they so readily undergo, is well known by chemists, as a strong obstacle to the separation of the volatile from the fixed parts of a com- pound by fire. Accordingly, in several processes, we are directed to add to the fusible compound, some porous substance, which is incapable of fusion, and will retain the whole in a spongy form, thereby to facilitate the dissipation of the volatile parts. In order to know whether an alkali would lose a part of its air, and acquire a degree of causticity, when ex- posed, with this precaution, to the action of a strong fire,